A schematic structure of a conventional rotation angle sensing device is shown in FIG. 6. The rotation angle sensing device includes a rotor J3 and a stator J6, each of which is made of magnetic material. The rotor J3 is divided in its diametral direction and has a substantially cylindrical shape. In magnet alignment gaps J1 of the rotor J3, which are positioned in divided parts of the rotor J3, magnets J2 are aligned so that they are magnetized in the same direction. The stator J6 is disposed inside the rotor J3. Moreover, the stator J6 is divided in its diametral direction and has a substantially cylindrical shape. In a magnetic sensing gap J4 of the stator J6, which is positioned in a divided part of the stator J6, magnetic sensing elements J5 (for example, hole ICs) are aligned.
When a relative rotation angle between the magnets J2 and the magnetic sensing elements J5 is changed, density of magnetic flux (magnetic force) flowing through the magnetic sensing element J5 is changed, and thereby output signals of the magnetic sensing elements J5 are changed. That is, the rotation angle sensing device is for detecting the relative rotation angle between the magnets J2 and the magnetic sensing elements J5 on the basis of the output signals of the magnetic sensing elements J5.
Such the rotation angle sensing device is, for example, described in JP-A-2001-317909, JP-A-2001-4315, JP-A-2001-91208 and JP-A-2001-289609. In the rotation angle sensing device, a typical relation between a rotation angle and density of magnetic flux passing through the magnetic sensing elements J5 is shown in FIG. 2A. The magnetic flux density turns back at ±90°. Therefore, detectable range of the rotation angle is between ±90°.
Generally, a permanent magnet is employed as the magnet J2. The permanent magnet has a characteristic that the amount of its generative magnetic flux changes in accordance with temperature. However, in a position where the magnetic flux density is detected as 0[mT], the detected magnetic flux density is the most unchangeable in accordance with the temperature. Therefore, in a position where the magnetic flux density is detected as 0 [mT], the magnetic flux density can be detected with the highest accuracy.
Moreover, when the rotation angle sensing device is used for detecting an opening degree of a throttle valve, a position where the magnetic flux density is detected as 0 [mT] needs to be used as the 0° position of the throttle valve so as to accurately detect a very small opening degree of the throttle valve in an idling state. Accordingly, the detectable range of the rotation angle of the throttle valve is limited between 0° and 90°, and a rotation angle more than 90° can not be detected.
That is, not only employing for the throttle valve, in the conventional rotation angle sensing device, when the 0° position of the rotation angle is set so as to correspond to the position where the magnetic flux density is detected as 0 [mT], it becomes impossible to detect the rotation angle more than 90°.
Moreover, it is considered that an outer device that generates magnetic force, such as a motor, is disposed around the rotation angle sensing device. In this case, by the effect of the outer device, the density of the magnetic flux flowing through the magnetic sensing elements J5 is changed, and thereby the detection accuracy of the rotation angle sensing device is disadvantageously lowered.
That is, the rotation angle sensing device is likely to be influenced by the magnetic force from the outside, and thereby the detection accuracy is likely to be lowered.